This invention relates to a process for the removal of a cyclic urea reaction product in an amine gas treating process which involves an absorption and a regeneration system. More particularly, the invention is concerned with the flash crystallization and filtration of a cyclic urea degradation product which forms as a by-product in the gas scrubbing process.
It is well known in the art to treat gases and liquids, such as mixtures containing acidic gases including CO.sub.2, H.sub.2 S, SO.sub.2, SO.sub.3, CS.sub.2, HCN, COS and oxygen and sulfur derivatives of C.sub.1 -C.sub.4 hydrocarbons with amine solutions to remove these acidic gases. The amine usually contacts the acidic gases and liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The acidic scrubbing processes known in the art can be generally classified into three (3) categories.
The first category is generally referred to as the aqueous amine process where relatively large amounts of amine solution are employed during the absorption. This type of process is often utilized in the manufacture of H.sub.2 and/or ammonia production where nearly complete removal of the acid gas, such as CO.sub.2 is required. It is also used in those instances where an acid gas, such as CO.sub.2, occurs with other acid gases or where the partial pressures of the CO.sub.2 and other gases are low.
The second category is generally referred to as the aqueous base scrubbing process or "hot potash" process. This type of process is generally used where bulk removal of an acid gas, such as CO.sub.2, is desired. This process also applies to situations where the CO.sub.2 and feed gas pressures are high. In such processes, useful results are achieved using amine activators in the aqueous potassium carbonate solutions.
A third category is generally referred to as the non-aqueous solvents process. In this process, water is a minor constituent of the scrubbing solution and the amine is dissolved in the liquid phase containing the solvent. In this process up to 50% of amine is dissolved in the liquid phase. This type of process is utilized for specialized applications where the partial pressure of CO.sub.2 is extremely high and/or where many acid gases are present, e.g., COS, CH.sub.3 SH, and CS.sub.2.
The present invention relates to a process for the selective separation of a cyclic urea degradation product which may form as a by-product of the practice of the second category of acid scrubbing processes described above. More specifically the invention relates to the aqueous base scrubbing process or "hot potash" process in which a hindered amine is used.
Many industrial processes for removal of acid gases, such as CO.sub.2, use regenerable aqueous alkali scrubbing solutions, such as an amine and potassium carbonate which are continuously circulated between an absorption zone where acid gases are absorbed and a regeneration zone where they are desorbed, usually by steam-stripping. The capital cost of these acid scrubbing processes is generally controlled by the size of the absorption and regeneration towers, the size of the reboilers for generating stripping steam, and the size of the condensers, which condense spent stripping steam so that condensate may be returned to the system to maintain proper water balance. The cost of operating such scrubbing plants is generally related to the amount of heat required for the removal of a given amount of acid gas, e.g., thermal efficiency, sometimes expressed as cubic feet of acid gas removed per pound of steam consumed. Means for reducing the costs in operating these industrial processes have focused on the use of absorbing systems or combinations of chemical absorbants which will operate more efficiently and effectively in acid gas scrubbing processes using existing equipment.
It is disclosed in U.S. Pat. Nos. 4,112,050; 4,112,051 and 4,112,052 that sterically hindered amines unexpectedly improve the efficiency, effectiveness and cyclic working capacity of the acid gas scrubbing processes in all three of the above-mentioned process categories. In the case of the sterically hindered amine activated "hot potash" CO.sub.2 containing acid gas scrubbing process of the invention described in U.S. Pat. No. 4,112,050, the process can be operated at a cyclic working capacity significantly greater than when diethanolamine or 1,6-hexanediamine is the amine activator used in a similar process. It is believed that the increase in cyclic capacity observed with the sterically hindered amines is due to the instability of their carbamates. In that respect, sterically hindered amines are similar to tertiary amines. However, tertiary amines usually are not used on a commercial scale for carbon dioxide containing acid gas scrubbing due to their low rates of absorption and desorption.
N-alkyl alkylene diamines are advantageously used as sterically hindered amine activators in the "hot pot" process. A preferred sterically hindered amine used as an activator in the "hot pot" process is N-cyclohexyl-1,3-propanediamine (CHPD). This amine in the presence of an amino acid is sufficiently water soluble under absorption and desorption conditions to maintain a single phase and it also has a very high absorption capacity.
Although N-cyclohexyl-1,3-propane diamine has been found to produce excellent results as an activator in the "hot pot" treating process, one drawback in processes where it has been used is that it produces a cyclic urea product when the acid treated gas is rich with CO.sub.2. The cyclic urea has a deleterious effect on CO.sub.2 removal rates and must be removed and replaced with fresh N-cyclohexyl-1,3-propanediamine. The makeup rate for the hindered amine has a minimal effect on the process economics; however, the cyclic urea that is formed must be selectively removed in order to be able to maintain acid gas removal performance.
Previously, it had been found that cyclic urea may be converted back to the corresponding diamine by maintaining the scrubbing solution at an elevated temperature for an extended time. While this process may be acceptable for scrubbing solutions having H.sub.2 S present, the rate of cyclic urea hydrolysis to the corresponding diamine is much lower in the absence of H.sub.2 S. Further elevating the temperature to increase the hydrolysis rate is not practical in a low H.sub.2 S environment, since this may cause decomposition of scrubbing system components. The addition of hydrogen sulfide to the scrubbing solution usually is not advised, since this may act as a poison to catalysts in subsequent processing steps.
U.S. Pat. No. 3,124,612 is directed at a method for crystallizing urea from a concentrated solution to recover fine crystals of urea of improved purity. This patent discloses the use of a scraped surface heat exchanger for deposition of the crystals thereon. This patent does not disclose the use of a vacuum crystallization zone or that the use of a vacuum crystallization zone can improve the filtration properties of the crystallized material over that produced by scraped surface chillers.
U.S. Pat. No. 3,102,908 is directed at the continuous crystallization of adipic acid utilizing a vacuum crystallizer having separate evaporation and crystallization zones. The addition of a liquid polyorgano siloxane is required to reduce the build-up of crystals and to reduce the tendency of the solution to foam.
U.S. Pat. No. 4,183,903 is directed at the discovery that cyclic ureas can be used as anti-foaming agents.
U.S. Pat. No. 4,180,548 and U.S. Pat. No. 4,292,286 are directed at the discovery that cyclic urea can be selectively precipitated from a scrubbing solution by cooling the solution to a temperature range of about 90.degree.-180.degree. F. utilizing a heat exchanger. Since the cyclic urea precipitates on the cold surfaces of the heat exchanger and inhibits further heat transfer for a continuous process, either a scraped surface heat exchanger must be used or a plurality of heat exchangers must be used with alternate service and regeneration cycles. Such a gradual cooling process does not permit accurate control over the crystal size and shape. Generally, the crystals formed from such a process are small and tend to inhibit subsequent filtration. Moreover, scraped surface heat exchangers tend to produce very fine crystals of cyclic urea which are difficult to filter.
Accordingly, it is desirable to provide a process in which the cyclic urea can be separated from the scrubbing solution irrespective of whether hydrogen sulfide is present in the scrubbing solution.
It is also desirable to provide a process in which the crystals of cyclic urea produced are readily separable from the scrubbing solution by filtration.
It is also advantageous to provide a process in which the filter cycles for separating cyclic urea are longer than those currently achievable.
It is also desirable to provide a process in which the cyclic urea separated contains reduced amounts of other scrubbing solution components to permit recovery, sale or disposal of the cyclic urea without the loss of substantial amounts of other scrubbing system components.